Surface acoustic wave (SAW) devices are fabricated by depositing an array of input electrodes and an array of output electrodes on the surface of a piezoelectric crystal such as a quartz crystal. The arrays of electrodes have the form of a line array which transmits sonic energy in the end fire direction along the surface of the crystal. The SAW device may be used as a filter or as a delay line with one array of electrodes being coupled to an input terminal and the other array of electrodes being coupled to an output terminal.
There is considerable interest in the utilization of SAW devices in the construction of oscillator circuits for use as a high fidelity signal generator. The delay of the delay line, or phase shift of the filter, of the SAW device is utilized to provide the necessary phase shift in the oscillator circuit for producing oscillation at a desired frequency. A SAW device is frequently utilized in an oscillator circuit for situations wherein it is desired to generate a signal having a relatively low noise content.
A problem arises in that presently available SAW device generate self-noise which places an upper limit on a signal-to-noise power ratio of a signal generated by a circuit employing the SAW device. It is recognized that a piezoelectric crystal is composed of molecules which are uniformly positioned in the crystal lattice structure. Such uniformity continues until the crystal surface whereupon a discontinuity in the molecular arrangement is found, the discontinuity involving polar molecules which display an affinity for polar water molecules. Thus, typically, there is found a loose bonding of hydroxyl ions to the quartz molecules along the crystal surface. It is believed that, in response to thermal agitation, the hydroxyl ions may shift from one bonding point to another bonding point along the lattice structure of the crystal surface. The breaking and making of the bonds during such jumping appears as a self-induced noise, sometimes referred to as flicker noise, which limits the short-term stability of the SAW device. The noise is observed at the terminals of the SAW device, and is manifested both electrically and mechanically because of the piezoelectric nature of the crystal.